Cables for power, electronic (telephone) transmission, hydrophone cables for oil exploration at sea and other various uses have been filled with various substances in order to protect against water intrusion since 1970. Intrusion occurs when water penetrates into a localized opening in a cable sheath and is free to channel as far as physical processes for water spread and transport allow, often hundreds of feet. Not only does this upset capacitance balance of the transmission cable line but it introduces more potential corrosion sites in proproportion to the length of wire that is wetted. The useful life of water-soaked wires is obviously shorter than that of dry wires.
The solution that has been widely adopted is to fill the voids in the cable with a water insoluble filling material that simply encapsulates the cable components to prevent water intrusion. However, although this physical function of the cable filling material is straight-forward, the choice of the material is not. Among the many considerations that are important for materials used in this application are the hydrophobic nature of the material, low temperature properties, flow characteristics at elevated temperatures, the highest temperature at which the encapsulant may be used ("upper service temperature"), processing characteristics, handling characteristics, dielectric properties, toxicity and cost.
Materials that satisfy most of these criteria and which have been widely used are described in U.S. Pat. Nos. 3,607,487 and 3,717,716 issued Sept. 21, 1971 and Feb. 20, 1973 respectively. These materials are essentially a petroleum jelly, mixed with a polymer, usually polyethylene, to impart consistency and prevent flowing at warm temperatures below the upper service temperature.
Similar hydrophobic encapsulants have been proposed for filling splice closures. For example, U.S. Pat. No. 3,879,575 issued Apr. 22, 1975 describes a mixture of a low viscosity oil gelled by a styrene-isoprene-styrene copolymer, again with polyethylene added to impart consistency and reduce slump.
U.S. Pat. No. 4,259,540 discloses the use of a sytrene-ethylene butylene-styrene block copolymer, polyethylene, and a paraffinic or napthenic oil, where the oil has a maximum of 5% aromatic oils, in order to enable the cable encapsulant to meet the functional requirements of the cable and to provide good handling characteristics that the petroleum jelly material does not possess.
However, all of these above-described encapsulants during the filling process, must be heated to a temperature above the "upper service temperature" of the encapsulant. The upper service temperature of the material is the temperature above which the material cannot be used in the cable. Thus, filling of most materials requires a time consuming step in order to heat the encapsulant to a pumpable consistency for filling. There has been long felt need for an encapsulant which meets other filling material requirements but which could be processed into the cable well below the "upper service temperature" of the encapsulant. Previously used encapsulants required that it and the cable be heated to a temperature well above the melting point of the encapsulant (i.e., the upper service temperature) in order to significantly reduce the viscosity of the fluid to allow filling of the cable. This heating is energy intensive and may be damaging to some of the electrical components of the cables. It also precludes the use of desirable cable component materials that may be advantageous, but which cannot tolerate high filling temperatures of these encapsulants.
In addition, an encapsulant which is thermally reversible has long been sought. This means that the encapsulant may be removed and replaced during maintenance time and time again at a temperature below the temperature that would damage the cables. An encapsulant which is thermally reversible can be heated to a liquid and then cooled to a gel over and over again without damage to the nature of the filling material, or cable components. This is especially true in hydrophone cables that are generally not permanently installed but towed at sea where the utility of such invention is paramount.